Production throughput and regularity are two of the most important key performance indicators in an oil and gas production system. Production throughput is intended to mean the oil and/or gas and/or water and/or liquid and/or total mass production per time interval (or, flow rate(s)), whilst regularity is intended to mean the production system's ability to meet the demands and quality requirements for intermediate or final product deliveries. The throughput and regularity depend on many different factors, some may be specific to each production system, others more general. One important and general factor in any oil and gas production system is how flow rate variations are mitigated or smoothened throughout the system. This is especially important when large flow rate disturbances are entering processing facilities in the production system and are directly connected to liquid levels and gas pressure control in the processing facility's buffer tanks. The structure and tuning of the associated control method directly affect the production system's throughput and regularity. By tuning it is meant the choice of parameters in the algorithms which constitutes the control method. Examples of buffer tanks or vessels or drums include, but are not restricted to, two/three phase separators, slug catchers, degassing drums, coalescers, inline degassers, inline deliquidisers, and scrubbers.
Disturbances in terms of variations in the oil, water, liquid, and/or gas flow rates entering the buffer tanks often cause problems for the liquid level and gas pressure controllers. The disturbances may be the results of                Terrain or riser induced slugging, also called severe slugging, see e.g. WO 02/46577.        Hydrodynamic slugging, that is, high-frequency slug flow as results of too large difference in the gas and liquid velocities.        Pigging of the pipeline. Pigging is an operation that is applied to pipelines for several reasons, such as inspection, application of chemicals like corrosion inhibitors, removal of solids or liquids, and so on. The pig is a mechanical device that is placed in the pipeline and is transported through the pipeline driven by the pressure difference and/or by a local motor. The pig sweeps up the liquid as it progresses through the pipeline and thus slug flow ensues.        Operational changes, such as switching between wells with different characteristics (for example different gas-oil ratios, water-cuts, etc.), or changes in well choke openings.        
Trips or complete or partial un-planned shutdowns often result due to such disturbances. Avoiding such situations are of great importance. In addition, in order to maximize the throughput and regularity of the production system, the flow rate variations throughout the system should be kept as small as possible by, for example, the buffer tanks' control system. This to not upset the processing facility more than necessary and in order to fulfill the quality requirements on processed oil, water, and gas. A typical example of a tightly tuned level controller (LIC) is shown in FIG. 17. This level controller aggressively tries to maintain a constant liquid level. The implication is that there is no mitigation of the disturbances entering the buffer tank. This again might cause problems for downstream processing units and equipment. It should be noted that large flow rate variations in the processing facility do not necessarily have to be caused by large flow rate variations entering the processing facility. It might be the result of poor tuning and/or unfortunate structure of the buffer tank controllers. This is illustrated in FIG. 18, where the level controller (LIC) amplifies the variations in the liquid flow entering the buffer tank.
At first glance, the two examples shown in FIG. 17 and FIG. 18 might seem easy to prevent by just performing a retuning of the controllers. Usually, the level and gas controllers are ordinary linear PID (Proportional+Integral+Derivative) controllers, which may be described by the following equation:
                    u        =                              K            P                    ⁡                      (                          ⅇ              +                                                1                                      T                    i                                                  ⁢                                                      ∫                                          T                      i                                                        ⁢                                                            ⅇ                      ⁡                                              (                        τ                        )                                                              ⁢                                          ⅆ                      τ                                                                                  +                                                T                  d                                ⁢                                  e                  .                                                      )                                              (        1        )            where u is the commanded valve opening, e is the control error (set point minus controlled variable measurement). Retuning means changing the controller parameters KP, Ti, and Td. This might of course improve the mitigation of the flow rate variations. However, the improvement will often be only temporary. That is, when the operating conditions change, the controllers with the new controller parameters will again perform poorly. This is due to complicating effects such as nonlinearities as, for example, variable process and valve gains, and interactions/couplings between control means and controlled variables, which a stand-alone plain PID controller is not designed to handle.